Research On Coupled Pore Fluid Flow And Stress In Injection-production Process Of Tight Sandstone Reservoir

Reservoir rock is widely considered to be elastic-plastic porous media,it's deformation behavior can be characterized by the effective stress principle,which shows that total stress of the reservoir is shared by both effective stress of rock skeleton and pore pressure of the reservoir pore fluid.In the process of reservoir injection-production,due to the change of pore fluid pressure,the effective stress of reservoir rock skeleton will change and then lead to changes of reservoir physical parameters,especially changes of porosity and permeability.In the meantime,the changes of reservoir physical parameters have an effect on the pore fluid flow and distribution of pore pressure.Therefore,there is an interaction between the deformation of rock and flow of it's internal pore fluid.This interaction is the dynamic coupling of pore fluid flow and stress,also known as the fluid-solid coupling interaction of the reservoir.At present,a large number of tight oil reservoirs have been discovered in China.Among them,tight sandstone reservoirs are the most abundant in China and have the most extensive distribution.They have broad prospects for exploration and have been discovered in the Ordos,Junggar,Songliao,Bohai Bay,and Sichuan basins.The pore throats of tight sandstone reservoirs are extremely small.A large number of studies have shown that it's deformation volume has a much greater effect on the reservoir properties than conventional reservoirs.Hydraulic fracturing and water injection to supplement the reservoir energy are almost the necessary stimulation measures for the development of tight sandstone reservoirs.During those measures of increasing production and development,the expansion of the reservoir sandstone skeleton and the rupture and extension of hydraulic fractures have greatly aggravated the influence of reservoir deformation on the reservoir property parameters,witch means that there is the more significant fluid-structure interaction during the development of sandstone.In addition,as the development time prolongs,the fluid-solid coupling interaction effect shows a more significant change,which will lead to changes in the physical properties of the reservoir,especially the changes in porosity and permeability are more pronounced,and the porosity,permeability,and deformation ability of the rock is directly related to the volume ofoil and gas production.Therefore,in order to predict the changes of parameters in the future,simulate the fluid flow in the reservoir accurately,then predict the reservoir development process correctly,and reveal the distribution law of the reservoir seepage and stress fields,we must consider the dynamic coupling between the change of seepage state of reservoir pore fluid caused by water injection and exploitation and the change of reservoir rock skeleton stress state and reservoir deformation.Based on a large number of previous experiments and theoretical research results,this paper has completed the following research work on the fluid-solid coupling theory of tight sandstone reservoirs:(1)Reservoirs are generally considered as porous media,which can be regarded as a structure composed of pores between the solid skeleton and the skeleton.Terzaghi effective stress principle is used to realize the coupling between the fluid seepage and deformation of porous media.Therefore,based on the principle of effective stress,the mechanical mechanism and fluid-solid coupling process of reservoir fluid-solid coupling interaction are studied.And the elasto-plastic deformation mechanism of rock skeleton during water injection is discussed.In the process of development,the shrinkage deformation mechanism of rock frame is studied.Based on the mechanism of fluid-solid coupling interaction,the mechanism of inter-layer interference between multi-oil reservoirs and general water injection was studied,and the influencing factors of interlayer interference were analyzed.(2)Based on the principle of effective stress,classical seepage mechanics theory and rock frame constitutive relations,the basic equations and boundary conditions of reservoir rock skeleton deformation and reservoir seepage theory models are summarized.The basic equations for the mathematical model of reservoir rock skeleton deformation include equilibrium equation,geometric equation and rock skeleton constitutive model.The basic equations for the seepage theory model of tight sandstone reservoirs include fluid continuity equations and fluid flow equations.(3)Summing up the existing research results of the dynamic model of reservoir property parameters,the dynamic evolution model for the change of porosity with volume strain,reservoir temperature,and pore pressure during the injection and production of tight sandstone reservoirs under fluid-solid interaction was deduced and established for the first time.At the same time,the dynamic evolution model of permeability with volume strain,reservoir temperature and pore pressure was established.(4)Explained the basic principles of the nonlinear finite element method for fluid-solid interaction problems.Combined with the basic equations of the reservoir rock skeleton deformation and reservoir seepage theory model,the derivation process of the transient finite element solution of the fluid-solid coupling problem is reviewed.This formula is the mathematical foundation of this paper based on ABAQUS simulation,and discusses the basic steps of the ABAQUS finite element software platform to solve the equation.(5)Finally,based on the ABAQUS finite element platform derived the dynamic evolution model of reservoir physical properties,the secondary development of field variable user subroutines was carried out,and fully coupled numerical simulation of fluid-solid coupling for the development process under different development modes.The simulation results accurately describe the dynamic process of seepage field,strain field,reservoir porosity,reservoir permeability,and production index during the development of the reservoir.The fluid-solid coupling model,theoretical research methods and the numerical simulation methods based on ABAQUS finite element software given in this paper can be used to analyze the change law of stress,rock strain,porosity,permeability,reservoir pressure,etc.over time and space in the development of tight sandstone reservoirs,as well as it's influence on reservoir seepage and production dynamics,which has a certain reference value for more accurately simulating the development of tight sandstone reservoirs,then guiding the long-term planning of reservoir development and adjustment of production plans to achieve stable production.